Abstract

We examined the physiological basis of backward masking by recording evoked magnetic fields with a whole-head MEG-system during a recognition task. On each trial, a digitised image of a natural scene was displayed, immediately followed by a noise mask. Subsequently, the target and a distractor were shown, and subjects had to indicate the target. At 92 ms and 37 ms of presentation time, recognition performance was 97 and 67 correct, respectively. The MEG data revealed that, in the first 80 - 120 ms, activity was concentrated over the occipital cortex. At the 92 ms target duration, the mask had no effect on the initial activity caused by the target. However, at 37 ms of target duration, processing of the mask briefly interfered with the target. A significant difference in MEG activation in correct and false trials occurred at about 160 ms after target onset. The results indicate that, during the first 40 ms of processing of a natural scene, new information arriving in the early visual areas can lead to a profound degradation of recognition performance, correlating with the temporal overlap of target and mask signals in occipital cortex. Later processing stages, beyond 180 ms, seem to be unaffected by the mask.